Why is my pitbull limping on her back leg after running, no pain?

Hi and thanks for using Petco Pet Education Center, formerly Petcoach! Sorry to hear about Storm. Just by the description an ACL injury (Anterior Cruciate Ligament) would be suspected but there are other causes also including hip or other joint problems (arthritis, degenerative joint disease), trauma, puncture wounds, fractures, soft tissue injuries to tendons/ligaments, muscle injuries, tick transmitted diseases, etc. ACL injuries usually occur when dogs are chasing squirrels, cats, birds, etc across the yard. They are fine and then they return lame, holding up the leg or bearing partial weight. That ligament when examined microscopically is undergoing a degenerative process in almost all of the cases. The strain on the ligament eventually tears or rips it completely. That same process can occur in the other knee and that's why this is so important to diagnose properly. Some dogs tear one ACL, get it fixed and then tear the other one. There are multiple ways to fix an ACL rupture or tear and I would discuss these with your veterinarian. Some of the newer techniques are specialized procedures only certain vets perform and they require that hardware (plates, screws) be put into the area to stabilize the knee joint. One older technique is called the lateral imbrication technique and I still think it works well if an experienced surgeon does it. It is much cheaper also compared to the newer procedures. There are some braces out now also for dogs that are unable to have a surgical repair done but I don't have any experience with those.

So I would advise taking Storm to your vet if this condition persists, worsens, or other signs develop. They can evaluate her, gather a history, and then make a diagnostic plan, and eventual treatment plan. A physical exam may include sedation/anesthesia and x-rays to fully assess the knee. Good luck with Storm.